Inspection of the pressure records from this cruise revealed several negative values (by up to 3 dB) at the start of some profiles. Reasonable temperature and salinity values accompanied the negative pressures, which casts some doubt on the accuracy of the pressure calibration. The affected data values have been flagged as suspect.

Open Data supplied by Natural Environment Research Council (NERC)

Bissett-Berman 9060 Salinity Temperature and Depth

The B-B 9060 STD measured salinity, temperature and depth and whose analogue output was in graphical form. The plots created were of salinity and temperature versus depth which had to be manually digitised.

RRS Challenger 8/76 STD Data Documentation

Introduction

Caution

Inspection of the pressure records from this cruise reveals several negative values (by up to 3db) at the start of some records, (while the temperature and salinity values appear reasonable), thus casting some doubt on the accuracy of the pressure calibration. These data values have been flagged.

Note that the sea floor depths for C876/036 and C876/041 (BODC Series Reference 91512 and 91573) are given as 47m and 70m respectively, these are less than the maximum depth attained by the STD by more than 10 per cent.

Instrumentation

The instrument used was a Bissett Berman STD system and the data were logged on a Hewlett Packard 9820. Instrument lowering and raising speeds between 0.5 m/s and 1 m/s. An acoustic pinger was placed above the STD to give an accurate depth measurement, this could then be used to check the STD pressure calibration. An NIO bottle with reversing thermometers was placed above the pinger, within 2m of the STD system. A bottle sample was taken at the bottom of the cast providing the temperature and salinity are uniform at that point. If large temperature or salinity gradients were present then the bottle sample was triggered at a suitable site on the upcast. A surface salinity sample was also taken at the start of the dip.

Calibration

The STD was not calibrated in the laboratory. The manufacturer's calibration was used and water samples taken to check the calibration and apply corrections where necessary.

Temperature

The manufacturer's calibration was used to convert the raw data to physical units using the equation below:

Temperature (°C) = (10 6 /Pt - 2238.68)/55.84 where Pt is the temperature period in microseconds

These values were then plotted against the water bottle (i.e. reversing thermometer) temperatures and a regression line fitted to the data such that:

Temperature(WB) = m x Temperature(STD) + c

Then the regression coefficients (m and c) were applied to correct the STD temperature data - these are given in the table below.

Salinity

The manufacturer's calibration was used to convert the raw data to physical units using the equation below:

These values were then plotted against the STD salinity and a regression line fitted to the data such that:

Salinity(WB) = m x Salinity(STD) + c + (m1 x Pressure + c1)/1000

Then the regression coefficients were applied to correct the STD salinity data. The data showed a salinity - pressure dependency, this was corrected by plotting delta-S (i.e. calibrated salinity - calculated salinity) against pressure to determine the slope (m1) and intercept (c1). These were then incorporated into the equation as shown above. The regression coefficients are given in the table below.

Pressure

The depths from the acoustic pinger were noted where the bottle samples were taken and then used to check the calibration of the pressure sensor - unless calibration values were available from the reversing thermometers. The equation below was used to convert the pressure period to physical units.

Pressure = (10 6 /Pd - 9712)/0.26267 where Pd is the pressure period in microseconds

A regression fit was carried out using the calibration values and the slope and intercept determined. The pressure values could then be corrected using:

Pressure (CORR) = m x Pressure(STD) + c

The fit of the STD data to the water bottle calibration data is given in the table below:

Variable

Slope (m)

Intercept (c)

Standard Deviation

Temperature (°C)

0.9949

0.0891

0.008

Salinity (ppt)

1.0060

-0.2831

0.022

Pressure (dbar)

0.4969

-4.5858

3.684

Delta-S

-0.0122

29.8937

8.545

Data processing

Obvious wild points were edited out of the calibration file and the calibration programs run to obtain values for the slopes and intercepts for temperature, pressure and salinity. These were then applied to the uncalibrated data and sigma-t was calculated. The data values were then sieved to ensure a minimum separation between pressure values of 1 dbar. The data were then visually inspected and major spikes flagged.

Fixed Station Information

Station Name

Ellett Line/Extended Ellett Line Station R

Category

Offshore location

Latitude

57° 0.00' N

Longitude

9° 0.00' W

Water depth below MSL

135.0 m

Ellett Line/Extended Ellett Line: Station R

Station R is one of the fixed CTD stations, which together form The Extended Ellett Line. The line lies between Iceland and the Sound of Mull (Scotland) crossing the Iceland Basin and Rockall Trough via the outcrop of Rockall. As part of this initiative, CTD dips, together with associated discrete sampling of the water column, have typically been carried out annually at this station since September 1996.

Prior to September 1996, Station R was part of a shorter repeated survey section, consisting of 35 fixed stations, known as The Ellett Line (originally termed the Anton Dohrn Seamount Section). This line incorporated those stations across the Rockall Trough and Scottish shelf between Rockall and the Sound of Mull and was visited at regular intervals (usually at least once a year) between 1975 and January 1996.

Fixed Station Information

Station Name

Ellett Line

Category

Offshore route/traverse

Ellett Line

The Ellett Line is a hydrographic transect consisting of 35 individual fixed stations which were occupied, usually at least once a year, between 1975 and 1996. The time series is named after the scientist David Ellett, who coordinated the survey work at Dunstaffnage Marine Laboratory (DML), near Oban. The transect ran between the north west coast of Scotland to the small outcrop of Rockall, via the Anton Dohrn Seamount - a prominent bathymetric feature in the Rockall Trough (see map). STD/CTD dips and associated water sampling for the analysis of nutrients were routinely performed during each station occupation.

In 1996 the transect was lengthened to incorporate new additional fixed stations crossing the Iceland Basin from Rockall to Iceland. This transect, which is still routinely occupied annually, is now known as the Extended Ellett Line and is a collaborative effort between scientists at Dunstaffnage Marine Laboratory and the Southampton site of the National Oceanography Centre (NOC).

Map of standard stations (1975-1996)

Map produced using the GEBCO Digital Atlas

The white triangles indicate the nominal positions of the Ellett Line stations (1975- 1996). Measurements made along the Ellett Line lie within a box bounded by co-ordinates 56° 40.02' N, 13° 42.0' W at the south west corner and 57° 37.2' N, 6° 7.98' W at the north east corner.

Nominal Ellett Line stations (1975-1996)

Listed below are nominal details of the standard hydrographic stations that formed the Ellett Line between 1975 and January 1996.